The present application claims the priority of British patent application No. 0104845,3, filed on Feb. 27, 2001. The disclosure of this prior related application is hereby fully incorporated by reference herein.
This invention relates to lamp assemblies, and more particularly to lamp assemblies for use in the printing and coating industry for the fast curing of inks and the like on a large variety of substrate materials.
It is well known to cure inks on a substrate by application of ultra-violet radiation from one or more medium-pressure ultra-violet lamps. It is also well known to provide each lamp in an assembly with a reflector which includes a reflective surface partly surrounding the lamp for reflecting radiation therefrom onto the substrate. The reflective surface has a concave profile which is commonly elliptical or parabolic, the lamp being mounted on the symmetrical centerline of the profile and adjacent the apex.
The reflector increases the intensity of the radiation received by the curable material. The penetration of the radiation into the material is an important factor in curing and, whilst penetration varies with different colors and materials, the higher the intensity the better the penetration.
One problem with known arrangements is that the angular spread of the radiation output from the reflector may be quite high with the consequence that radiation is received across a wide band of the substrate at varying levels of intensity. The highest intensity locations will depend on the degree of focusing provided in the assembly but there may be regions where the level of intensity is low. The large angular spread means that the substrate has to be moved more slowly than is desirable if the intensity of radiation is to be sufficiently high.
Another problem which arises with known arrangements is that part of the radiation is reflected back onto the lamp itself, which reduces the amount of radiation energy available for curing and leads to heating of the lamp which can adversely affect lamp operation and increase the already large amount of heat given off by the assembly which may cause warping and distortion of the coating and/or the substrate.
This problem has been recognized in French Patent 2334966 which describes a reflector in the form of two half-shells, each of which is pivotal about a longitudinal axis within the cavity to the sides of the symmetrical centerline thereof. The French Patent proposes deforming the top region of the reflector to give it, externally, a generally concave shape across the width of the lamp by bending the top edge of each half shell down towards the lamp.
The apparatus disclosed in French Patent 2334966 has disadvantages as a result of its basic form in that a complicated system will be necessary to achieve the desired pivoting action and space has to be provided to accommodate the half-shell pivoting which is inconsistent with the current industry desire for smaller curing assemblies. Cooling of the half-shells will be difficult, again because of the need to accommodate the pivoting action. Problems will also arise as a result of the solution proposed in the French Patent to the problem of lamp self-heating. The distortion of the reflector towards the lamp will lead to excessive heating of the distorted portion and will make cooling of the adjacent region of the lamp much more difficult.
The desire in the industry for smaller curing assemblies mentioned above gives rise to a problem in that decreasing the width of the assembly to enable it to occupy a smaller space in a line can have the result of increasing the angular spread of the emitted radiation. This in turn gives rise to the problems already discussed above.
The efficient and effective cooling of lamp assemblies has been a constant problem which has become even more important as ever increasing lamp powers have been employed to give faster curing such that substrate speeds can be increased. For example, at the date of the French Patent, 1975, lamp powers were only in the region of 250 Watts per inch (100 Watts per cm). Lamp powers of 200-400 Watts per inch (80-160 Watts per cm) are now common and lamps of even higher powers, 500-600 Watts per inch (200-240 Watts per cm) are increasingly being used. Furthermore, the advantages of UV curing, including cleanness and quality, have led to a demand for curing systems capable of operating with a wide variety of substrates, including substrates which are very vulnerable to heat damage.
Earlier assemblies were generally cooled by air alone. In the first air-cooled systems, air was extracted from within the reflector through one or more openings provided above the lamp to draw out the heat. In later systems, cooling air was blown into the assembly and onto the lamp, again through openings located adjacent the lamp. A problem with air cooling is that the blowers required increase the size of the assembly making it difficult to install between the stands of a multi-stand press.
This, and the increasing cooling requirements due to higher lamp powers, led to the use of water cooling alone or in conjunction with air cooling. The cooling water is fed through tubes attached to or integrally formed in the reflector. In addition, a number of designs have been proposed with filters comprising one or two tubes of quartz provided between the lamp and the substrate through which liquid is passed, typically de-ionized water. As well as contributing to the cooling, the filters have the primary effect of filtering infra-red radiation, which tends to heat the substrate, and focusing the light from the lamp onto the substrate. The liquid coolant is circulated to and from all the tubes through cooling or refrigerating means.
As lamp powers increase, ever more efficient and effective cooling systems are required to keep temperatures within acceptable limits, not only to prevent damage to the substrate, but also to prevent harm to adjacent equipment and to operators of the printing system.
One known design of lamp assembly has a reflector in the form of a block with a cavity on the surface of which the reflective surface is provided. The reflective surface may be formed by polishing the cavity surface or a specific reflector member can be attached thereto. In either case it is known to provide coatings on the reflective surface of heat-absorbing material.
British Patent No. 2315850 discloses a lamp assembly in which the reflector block is formed in two parts. The reflector surface is provided by two reflector plates, each of which is fitted between a flange extending into the cavity and a clamp attached to an end of the reflector block half by tightenable fastening means.
It is known to water cool reflector blocks by forming one or more passages therein for flow of cooling water. With two-part blocks, this requires water inlet and outlet pipes for both parts, that is, four pipes in total. The need to accommodate these pipes and to maintain the integrity of the water seals between them and the block passages makes the assembly as a whole unwieldy and furthermore makes it difficult to move one block part relative the other.
A further problem with block form reflectors, and indeed other reflectors, is that the radiation source is often relatively inaccessible and so it takes a significant time to change the source. This means that there may be significant down time when the lamp or other type of radiation source has to be changed.
It is a general object of the present invention to provide a lamp assembly which overcomes one or more of the problems associated with known assemblies, as discussed above. It is a more particular object to provide a lamp assembly which can be of small size but still provide high intensity radiation by reducing the angular spread of the radiation. It is a further particular object to provide a lamp assembly with a water cooling system, which has minimal equipment and is easier to accommodate in the assembly. It is a still further particular object to provide a lamp assembly in which the lamp or other radiation source can be readily accessed and so easily changed.
A lamp assembly in accordance with a first aspect of the invention comprises an elongate source of radiation, a reflector with an elongate reflective surface partly surrounding the source and having an opening for emission of radiation down towards a substrate for curing a coating thereon, the reflector comprising two body members each having a shaped surface which combines with the other when the body members are held in a first relative position to form a cavity in which the source is located and on the surface of which the reflective surface is provided, at least one passage through each body member for cooling water flow, and a tube for cooling water flow located in the vicinity of the emission opening wherein the or a passage in one body member is connected to the tube which is connected to the or a passage in the other body member.
The advantage of this is that only one water inlet tube and one water outlet tube is required, the outlet water from one body member being inlet to the other body member via the cooling tube. Thus the cooling tube is used as part of a flow path between the two body members and the number of water tubes is halved from four to two in comparison with known arrangements where the reflector is formed from two body members.
In accordance with another aspect of the invention, there is provided a lamp assembly comprising an elongate source of radiation, a reflector with an elongate reflective surface partly surrounding the source and having an opening for emission of radiation down towards a substrate for curing a coating thereon, the reflective surface having a generally concave profile and the source being located near the base of the concavity, wherein the reflector comprises two reflector elements each having a shaped surface which combines with the other when the elements are held in a first relative position to form a cavity in which the source is located and on the surface of which the reflective surface is provided, and wherein the source is mounted such as to be movable with one element to a second position relative the other element in which the source is located in a user accessible position.
This arrangement overcomes the problem found with lamp assemblies that a significant time is required to change the radiation source. By mounting the radiation source such that it is movable with one element of the reflector relative the other into a user accessible position, repairing or replacing the radiation source can be more quickly performed.
Preferably the reflector elements each comprise a body member having at least one passage for cooling water flow and the first and second aspects are combined with the passages in the body members being connected via a tube for cooling water located in the vicinity of the emission opening.
The combination is particularly efficient if the movable body member is pivotable relative the other body member about a pivot axis parallel to the longitudinal axis of the cooling tube. The cooling tube acts in effect as a rotary union and allows access to the radiation source without any potential adverse effect on the integrity of the water seals.
In a still further aspect, the invention provides a lamp assembly comprising an elongate source of radiation, a reflector with an elongate reflective surface partly surrounding the source and having an opening for emission of radiation down towards a substrate for curing a coating therein, the reflective surface having a curved generally concave profile between the edges of the emission opening which is symmetrical about a centerline on which the source is located, wherein the reflector has two elongate radiation diverting surfaces extending down from the edges of the emission opening and arranged to reflect radiation reflected by the reflective surface and divert it toward the centerline, thereby to reduce the angular spread of radiation reaching the substrate.
It has been found that by providing the radiation diverting surfaces extending down from the emission opening, it is possible to focus the radiation into a narrow beam which also has the effect of increasing the intensity of the radiation reaching the substrate. The provision of diverting surfaces is particularly useful when the width of the assembly as a whole has been reduced since, as discussed above, this may otherwise give rise to potential for wide angular spread and the problems which result therefrom.
The diverter surfaces may extend at an angle away from the centreline and may be flat or slightly curved. If so arranged, their primary effect is to turn radiation emitted from the lower sides of the source which would tend to be at a relatively large angle away from the centerline back in towards the centerline and so combine that radiation with the radiation emitted from the top and bottom of the source to give a focused beam of comparatively constant high intensity.
The reflector may comprise a body having a cavity in which the source is located and on the surface of which the reflective surface is provided and the diverter surfaces may be provided on separate end pieces mounted on the body. If the known arrangement whereby the reflective surface comprises at least one plate secured by a clamp on either side of the emission opening is adopted, then the clamps can act as the end pieces. Whatever form the end pieces take, they are suitably made of, or coated with, a reflective material, the first alternative being preferred.
All three aspects may be combined to result in a lamp assembly which can be small but still produce high intensity radiation of low angular spread whilst being water cooled by a single water inlet and water outlet tube. Furthermore the assembly is efficient in use since the radiation source can readily be accessed and so down time when the source needs to be repaired or replaced is minimized.